Generate a random unique "Request ID" for SSH requests so processes can coordinate better

Summary:
Depends on D19247. Ref T13109. When we receive an SSH request, generate a random unique ID for the request. Then thread it down through the process tree.

The immediate goal is to let the `ssh-exec` process coordinate with `commit-hook` process and log information about read and write lock wait times. Today, there's no way for `ssh-exec` to interact with the `PushEvent`, but this is the most helpful place to store this data for users.

Test Plan: Made pushes, saw the `PushEvent` table populate with a random request ID. Exported data and saw the ID preserved in the export.

Maniphest Tasks: T13109

Differential Revision: https://secure.phabricator.com/D19249

src/applications/repository/query/PhabricatorRepositoryPushLogSearchEngine.php

@@ -101,6 +101,9 @@ final class PhabricatorRepositoryPushLogSearchEngine
       $fields[] = id(new PhabricatorIDExportField())
         ->setKey('pushID')
         ->setLabel(pht('Push ID')),
+      $fields[] = id(new PhabricatorStringExportField())
+        ->setKey('unique')
+        ->setLabel(pht('Unique')),
       $fields[] = id(new PhabricatorStringExportField())
         ->setKey('protocol')
         ->setLabel(pht('Protocol')),
@@ -209,6 +212,7 @@ final class PhabricatorRepositoryPushLogSearchEngine
 
       $map = array(
         'pushID' => $event->getID(),
+        'unique' => $event->getRequestIdentifier(),
         'protocol' => $event->getRemoteProtocol(),
         'repositoryPHID' => $repository_phid,
         'repository' => $repository_name,